I like to think that I am a bit smarter than my camera, so I try to use the little grey cells rather than a grey card for every shot.
I have never thought a grey card was much help with exposure, after all, the things I photograph are mostly not grey or average in colour (grass is I understand a good average 18% grey if you take out the green channel), but the interesting parts of the image lay above or below that reading.
A good photog will add a bit of exposure comp if his subject is white and subtract if it is black. (please see my image of a dog under topic 1600 iso)
That said todays Matrix meters do most of that for you by having a data-base of thousands of shots in their memory to match what you are looking at and it will have a go at giving you the best exposure.
Most of the time this works and we don't have to think about it. But sometimes we do and it is good to know how your camera meter will react to the extremes of exposure, the blacks and whites that we come across.
Have a little experiment and see what your camera does with a white wall and how much compensation you have to dial in to make the wall white in your shot. Do the same with a dark subject.
If I hadn't known before hand that I needed a +1.0 exposure for my shot of the white puppy running he would have come out too underexposed, so I had it dialed in before I took the shot.
here is a short article that might help, using a D80 and a D90 to photograph white, black, and grey cards, as he says the D80/90 the most baffling of cameras, by which he means they have a tendency to overexpose most of the time then just when you weren't expecting it get it spot on. (Most D80 owners have a -0.7 dialed in all the time.)
http://esfotoclix.com/blog1/?p=218
If any of the newer readers want to ask questions please do, I promise I won't think any question stupid or too basic and I am sure the other photogs here will help too. There are hundreds of years of experience here please feel free to use it.